This invention relates to subsea pipelines.
Subsea pipelines are used to transport hydrocarbon fluids, such as oil or gas, across seas or oceans or from drilling stations to a shore location. Such pipelines generally comprise tubular sections which are joined together axially to form the pipeline. The joining can be carried out by welding or by a mechanical connector.
Subsea pipelines are now being used at considerable depths e.g. 2000 to 3000 feet below the surface of the oceans. A problem with such pipelines is damage which can occur at these extreme depths and particularly the remedy or repair of such damage.
There are alternative ways of dealing with the problem. One is to provide a replacement pipeline. This can be done at the time of laying the original pipeline i.e. provide a standby pipeline for use in the event of damage to the one being used. However, this is extremely costly and in any event the second standby pipeline is also vulnerable to damage and deterioration. If the second pipeline is not laid until the first is damaged, the result is loss of service for a considerable time. Such a repair method is thus unsatisfactory.
Two other possibilities are in situ repair or replacement of the damaged section which generally are less costly than the provision of a standby pipeline. Both require certain operations to be carried out at depth. These operations can be carried out by hyperbaric divers, by the use of manned submersibles or by the use of a remotely controlled vehicle linked to the surface by an umbilical depending on the depth. In general the depth capabilities are up to 1000 feet for hyperbaric divers, up to 3000 feet for manned submersibles and up to 6000 feet for unmanned submersibles.
In situ repair can be effected by welding or by installing a mechanical seal. Welding is costly and requires complex equipment and qualified operators. Secondly the welder is at sea-water pressure and thus welding can only be carried out at depths of 1000 feet or less. Mechanical seals can be installed by divers, by using manned submersibles or by remote-controlled vehicles. However, at extreme depths when using manned submersibles or remotely controlled vehicles it is difficult to obtain satisfactory seals and the method has serious limitations. Replacement of a damaged section is less costly than the standby pipeline method and can be carried out relatively quickly. Additionally it does not suffer from the depth restrictions of welding nor does it have the limitations of the mechanical seal method.